The invention discloses a large water turbine stay ring machining and centering method. The method includes: during drawing checking of a stay ring, using the central line of a fixed guide vane as the standard for leveling, using the internally tangent circle of the fixed guide vane as the standard to determine the center 0-0 of the stay ring, using 0-0 as the standard to draw the machining lines of the inner circles of an upper annular plate and a lower annular plate, using the circle center to measure radium, drawing a joint face machining line, allowing the joint face to face the spindle of a machine tool during joint face milling of the stay ring, leveling a workpiece according to a flat line, and machining the joint face according to the fact that the joint face is vertical to the spindle of the machine tool. The method has the advantages that the method is simple to operate and high in centering precision, the joint face interference is small during circle assembling after machining, production cycle can be shortened, machining quality is increased, and the like.